This application claims priority under 35 U.S.C. xc2xa7 119 to Korean Patent Application No. 2001-10972, filed on Mar. 3, 2001, the entirety of which is hereby incorporated by reference herein for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a storage electrode and a method for fabricating the same, and more particularly to a high storage electrode having a stable structure which can be prevented from falling down when installed in a limited cell area, and a method for fabricating the same.
2. Description of the Related Art
Recently, as the integration degree of a memory device, such as a DRAM, increases various methods are studied and developed for obtaining sufficient capacitance in a limited cell area. In order to increase the cell capacitance, a material having a high dielectric constant is used as a capacitor dielectric layer, an effective area of a cell capacitor is enlarged by growing an HSG (hemisphere silicon grain), and/or a high dielectric material is used as a dielectric layer.
Besides the HSG method, in which the effective area is enlarged by a concave and convex surface structure, a technique for raising the height of a storage electrode layer forming the cell capacitor is introduced in order to enlarge the effective area of the cell capacitor. Since the HSG process is complicated and increases the manufacturing cost, techniques for raising the height of the storage electrode of a capacitor are being studied and developed.
In order to obtain sufficient cell capacitance in a small cell area, a capacitor having a height more than 15,000 xc3x85 is introduced into the gigabit DRAM. As the cell area becomes smaller, the height of the capacitor is relatively increased so that the storage electrode is tilted or, in an extreme case, falls down.
Generally, in a cylindrical capacitor, an opening is formed in an insulating layer and a cylindrical storage electrode is formed along the profile of a bottom and a sidewall of the opening. The bottom of the opening formed in the insulating layer is relatively narrower than an inlet of the opening due to a loading effect created during an etching process. Accordingly, the sidewall of the opening is sloped.
Polysilicon is coated on the bottom and the sidewall of the opening along the profile of the opening, so that a polysilicon storage electrode layer has an unstable structure. That is, a bottom portion of the polysilicon storage electrode layer is narrower than an inlet portion of the polysilicon storage electrode layer. In addition, when a peripheral insulating layer is removed, the sidewall of the storage electrode layer has a reverse-inclination so that the storage electrode layer is tilted due to the weight thereof and easily falls down if an impact is applied thereto when a wafer is moved for the following process.
A method for fabricating a cylindrical storage electrode will be described with reference to FIGS. 1 to 3.
FIGS. 1 to 3 are sectional views showing the process steps for forming a cylindrical storage electrode layer of a semiconductor memory device.
Referring to FIG. 1, in order to provide a DRAM cell, a trench type field oxide layer 12 is formed on a silicon substrate 10 and an active device is formed in an active area. Generally, the active device is a metal oxide semiconductor (MOS) transistor.
The MOS transistor includes a gate electrode having a stacked structure of a polysilicon layer 16 and a tungsten silicide layer 18 formed on a gate oxide layer 14. The gate electrode is protected by a sidewall spacer 20 and a mask layer 32 which are comprised of insulating materials. Source and drain regions are formed by implanting impurities into surface portions of the active area of the silicon substrate 10 by using a gate electrode layer as an ion implanting mask.
Self-aligned contact holes are formed for the drain and source regions. Then, a conductive material, such as doped-polysilicon, is filled in the contact hole so as to form contact plugs 24 and 26.
Then, the MOS transistor is covered with an insulating layer 28 and the surface of the insulating layer 28 is polished by means of a chemical mechanical polishing (CMP) process.
After forming a bit line contact in the insulating layer 28 and exposing a drain contact plug 24, a bit line 30 is formed. The bit line 30 is protected by a sidewall spacer 32 and a mask layer 34 which are comprised of insulating materials.
The surface formed with the bit line 30 is coated with an insulating layer 36. Then, the surface of the insulating layer 36 is polished by means of the CMP process.
An opening is formed in the insulating layer 36 by a photolithography process. Then, a buried contact plug 37 is formed by filling polysilicon into the opening.
An etching stop layer 38 is formed by coating a nitride layer on the insulating layer 36. Then, an insulating layer 40 having a thickness of 15,000 xc3x85 is coated on the etching stop layer 38 and a photoresist pattern 42 for defining a capacitor forming area is formed thereon.
An opening 44 is formed by anisotropically and sequentially etching the insulating layer 40 and the anti-reflective layer 36 using the photoresist pattern as an etching mask. Accordingly, an upper surface of the buried contact plug 37 is exposed at a bottom 44b of the opening 44.
At this time, the bottom 44b of the opening 44 is narrower than an inlet 44a of the opening 44. That is, the bottom critical dimension becomes narrower than the critical dimension of the opening. Therefore, the sidewalls of the opening 44 are sloped. The reason is that the etching rate of the bottom is lower than the etching rate of the inlet due to the loading effect generated during the etching process. The loading effect extremely appears as the depth of the opening becomes deeper, that is as the height of the storage electrode layer becomes higher.
Referring to FIG. 2, after stripping the photoresist pattern 42 which is used as the etching mask, a polysilicon layer 46 is coated to a uniform thickness along a profile of the insulating layer 40 exposed through the openings 44.
Referring to FIG. 3, the polysilicon layer 46 formed on the upper surface of the insulating layer 40 is removed by performing an etch back process so that the polysilicon layer 46 is defined by each of the openings 44. Then, the remaining insulating layer 40 is removed so that the storage electrode layer having the bottom narrower than the inlet thereof is obtained as shown in FIG. 3.
However, the above storage electrode layer has a geometrically unstable structure due to the unbalanced structure of the inlet, the bottom and the sidewalls thereof, which is inclined so as to have a greater inlet than the bottom. For this reason, the storage electrode layer is tilted caused by the weight thereof, or, in extreme case, the storage electrode layer falls down thereby causing the failure of the cell.
If the storage electrode layer is tilted towards an adjacent storage electrode layer, a two-bit error may occur so that the reliability of a semiconductor device is lowered.
In addition, if the storage electrode layer falls down, the cell fails so that the yield rate of the semiconductor device is lowered.
Therefore, it would be desirable to provide a storage electrode of a semiconductor memory device having a geometrically stable structure, in which a base of the storage electrode layer is larger than a top of the storage electrode layer.
It would also be desirable to provide a method which is suitable for fabricating the above storage electrode.
The present invention has been made to address the above problems of the prior art. Other objects and advantages will appear hereafter.
In one aspect of the invention, there is provided a storage electrode of a semiconductor memory device. The storage electrode has a cylindrical base section having a first inner diameter and a truncated-conical xe2x80x9cpipe-shapedxe2x80x9d top section having a second inner diameter smaller than the first inner diameter. An upper end of the base section and a lower end of the top section extended towards each other at a same height so as to be connected to each other.
In another aspect of the invention, a storage electrode for a semiconductor device, includes a hollowed-out bottom section disposed on a substrate and a hollowed-out top section disposed on the hollowed-out bottom section, wherein a largest cross-section of the hollowed-out top section taken along a plane parallel to the substrate is smaller than a smallest cross-section of the hollowed-out bottom section taken along the plane parallel to the substrate.
Beneficially, a height of the base section is about 1,000 to 5,000 xc3x85 and a height of the top section is about 5,000 to 15,000 xc3x85. Beneficially, a height of the storage electrode layer is at least 15,000 xc3x85.
In yet another aspect of the present invention, there is provided a method for fabricating the storage electrode. A wafer having a first insulating layer formed with a buried contact plug is prepared. An etching stop layer is formed on the first insulating layer formed with the buried contact plug. A second insulating layer is formed on the etching stop layer with a predetermined thickness, and then an impurity is implanted into the second insulating layer. A third insulating layer is formed on the second insulating layer with a predetermined thickness. A photoresist pattern for defining a capacitor forming area is formed on the third insulating layer and then an opening is formed by anisotropically etching the third insulating layer, the second insulating layer and the etching stop layer, in sequence, by using the photoresist pattern as an etching mask in such a manner that an upper surface of the buried contact plug is exposed. After removing the photoresist pattern, a cleaning process is performed against the resulting structure such that the second insulating layer exposed through the opening is isotropically etched. After depositing polysilicon along a profile of the second and third insulating layers to a uniform thickness, the polysilicon deposited on an upper surface of the third insulating layer is removed. The remaining third and second insulating layers are removed to obtain the storage electrode of a semiconductor memory device.
Beneficially, the predetermined thickness of the second insulating layer is in a range of about 1,000 to 5,000 xc3x85 and the predetermined thickness of the third insulating layer is in a range of about 5,000 to 15,000 xc3x85.
Beneficially, Ar or N2 is implanted into the second insulating layer as an impurity. By implanting the impurity into the second insulating layer, the second insulating layer is more rapidly isotropically-etched than the third insulating layer. An SCl solution, an HF solution, or a mixed solution thereof may be used as a cleaning liquid in the cleaning step.
An anti-reflective layer may be formed on the third insulating layer.
The second and third insulating layers may be comprised of PE-TEOS.
Beneficially, instead of implanting the impurity into the second insulating layer, the third insulating layer having an etching rate smaller than an etching rate of the second insulating layer is used when an isotropic etching process is carried out. In this case, an isotropic etching rate of the second insulating film is greater than an isotropic etching rate of the third insulating film by a factor of about 1.5 to 3 times.